Charge coupled devices, hereinafter referred to as CCDs, are used as image sensors and shift registers and are typically comprised of sets of closely spaced electrodes, or gates, which usually are composed of doped polycrystalline silicon. Typically, close spacing between adjacent gates is necessary to assure satisfactory charge transfer efficiency. In order to achieve the small dimensions required for this spacing, a process is conventionally employed whereby a first set of electrodes is deposited and patterned. This first set of electrodes is then subjected to an oxidizing process where a thin oxide is formed to completely envelop the first set of electrodes. Then a second set of electrodes is deposited and patterned with the edges of the second set of electrodes overlapping the edges of the first set so as to form a closely spaced array of electrodes. The oxide which had been grown of the first level electrode serves then to insulate adjacent electrodes from one another while maintaining the desired close spacing. The material of choice for gate electrodes is generally chosen to be polycrystalline silicon because of its ability to be oxidized with a high integrity insulating oxide, whereas, the other conductive materials, such as indium tin oxide (ITO), which cannot be readily oxidized to form an insulating coating, are not suitable.
Devices with polycrystalline silicon electrode structures, as described above, are commonly employed in image sensing devices where light must first penetrate these electrodes, then be absorbed in the silicon substrate to form electron/hole pairs wherein the photogenerated electrons are collected and detected by appropriate circuitry. The sensitivity of such devices is, however, diminished by the overlaying polysilicon which absorbs some of the incident light before it reaches the silicon substrate. Devices have been proposed where the second electrode layer is replaced by a more transparent material such as indiumtin-oxide (ITO). See, for example, Kosman et al. IEEE International Electronic Devices Meeting, 1990. Technical Digest, p 287-290. This improves the device sensitivity but some light is still absorbed by the first layer electrode which is still composed of polysilicon and covers approximately 50% of imaging area. A so-called, virtual phase CCD with a single transparent electrode has been reported by Keenan et al. (IEE Trans. on Electron Devices ED-32, 1531, (1985)) which avoids this light absorption but that device is limited in its charge handling capacity due to the requirements of the virtual phase segments of the CCD register and, also by increased processing complexity.
In a multiphase CCD sensor, if the first level electrode as well as the second level electrode were composed of ITO, the sensitivity of the device could be improved while retaining the charge handling capabilities of a multielectrode CCD structure. However, most transparent conductive materials, such as ITO, are not amenable to oxidation for the formation of a high integrity insulator needed to insulate the first level electrode from the second level electrode. For satisfactory device operation it is necessary to insure against electrical leakage or short circuits between the closely spaced electrodes. A method for fabrication of a CCD device with a single layer of ITO with narrow etched spaces between electrodes has been proposed by Kosman et al. U.S. Pat. No. 5,114,872. This method is complex, involving deposition of at least three additional layers over the ITO layer and also depends critically on etch selectivities and anisotropy.
Wan et al., in U.S. Pat. No. 4,807,004, proposed a CCD device wherein a single ITO layer is deposited as a semi-insulative layer and then rendered conductive in selective areas by ion implantation. These electrodes are defined by narrow unimplanted regions in the ITO layer. The spacing between adjacent gate electrodes in such a device is limited by the resolution of the photomasking process used to define the unimplanted regions and by the leakage current which can be tolerated between the CCD clock phases via the unimplanted ITO regions. Obviously, the leakage currents will be dependent on the width of the unimplanted regions, with narrower electrode spacings, which are usually desired for good charge transfer efficiency, having higher leakage currents.
From the foregoing discussion, it should be apparent that there remains a need in the art for a method and apparatus employing transparent electrodes, whereby, sufficient insulation is achieved between electrodes without resulting oxidation in the electrodes. There is also a need for a simplification of the processes required to produce such electrodes.